1. Field of the Invention
The present invention relates to a signal transmission circuit for transmitting signals from an input terminal section to a processing circuit section, and an electronic equipment (device) provided with the signal transmission circuit.
2. Description of the Related Art
In related art electronic equipment, a signal transmission circuit for transmitting signals to a processing circuit section for signal processing is provided with a circuit for eliminating noise components of signals given from the outside of the electronic equipment to the electronic equipment (see, for example, Kenichi Ito, “Earth and Static Electricity”, THE NIKKAN KOGYO SHIMBUN, LTD., Jun. 30, 1980, pages 2 and 67). The noise component elimination circuit is realized by, for example, a low-pass filter constituted by a capacitor and a resistor. The noise component elimination circuit is placed right near an external terminal section.
Further, in another related art circuit board that has a constitution for increasing a noise elimination capability of a noise eliminating filter, at least one of a plurality of ground patterns for grounding is used as a ground pattern for the noise eliminating filter, and a grounding section of a circuit device right near a noise source in the noise eliminating filter is connected to the ground pattern for the noise eliminating filter (see, for example, Japanese Unexamined Patent Publication JP-A 5-235679 (1993)).
In still another related art noise component eliminating circuit in which a bypass capacitor is used, connecting the bypass capacitor in parallel between a power source line and a grounding line to which a noise source is connected is commonly known.
In still another related art control equipment in which at least two circuit boards, on each of which a circuit device is mounted, are placed substantially parallel, as a structure for eliminating noise, in the case where the two circuit boards are connected by the use of a connection jumper. A signal line which is apt to be affected by noise of a plurality of signal lines in the connection jumper is made of metal which has a specified resistance value higher than the resistance values of the other signal lines (see, for example, Japanese Unexamined Utility Model Publication JP-U 5-4590 (1993)).
In still another related art, a circuit for eliminating noise components using a constitution part in a printed circuit as a circuit device is proposed (see, for example, Japanese Unexamined Patent Publications JP-A 9-298368 (1997), JP-A 11-26907 (1999), JP-A 6-196836 (1994), JP-A 7-202374 (1995), JP-A 7-66519 (1995), JP-A 10-233562 (1998)).
In a ceramic wiring board of JP-A 9-298368, a via hole of an internal layer of the board formed by the use of a resistive element whose major component is RuO2 is used as a resistor. In a printed wiring board of JP-A 11-26907, via holes for connecting conductor patterns that exist in mutually different layers of the printed wiring board are formed so that each hole has a resistance value of 1 Ω or more, and the via holes are used as resistors.
In a memory module of JP-A 6-196836, a chip capacitor for cutting noise is realized by a printed part. The chip capacitor realized by a printed part is preformed by the use of a printing technique on one face of a board to which a memory is to be mounted. In a printed board of JP-A 7-202374, a capacitor as a capacitive element provided in a noise eliminating circuit and a coil as an inductive component provided in the noise eliminating circuit are realized by printed parts. The capacitor realized by a printed part is constituted by conductive patterns that are formed on both faces of a board so as to face each other via the board. In order to realize the coil by a printed part, a plurality of linear conductive patterns formed on both the faces of the board, respectively, are connected in series alternately via through holes so as to become one spiral conductive path on the whole.
In a flexible board of JP-A 7-66519, a trap filter part constituted by a pair of coils and one capacitor is formed by using a reactance component and a parasitic capacitance of a conductive pattern as a signal line on the flexible board. In a printed board of JP-A 10-233562, an LC noise filter that includes a capacitor and an inductor is formed as a printed circuit device. Therefore, the printed board has a constitution that signal transmitting wiring having a zigzag portion is placed on one face of the board, grounding wiring is placed on the other face of the board, and through holes are connected to the zigzag portion of the signal transmitting wiring and the grounding wiring, respectively. The through hole of the zigzag portion of the signal transmitting wiring and the through hole of the grounding wiring constitute the capacitor. The zigzag portion of the signal transmitting wiring constitutes the inductor.
FIG. 12 is a plan view of a signal transmission circuit 3 of a related art that comprises a capacitor 1 for eliminating noise components and uses a printed wiring board 2. This signal transmission circuit 3 is a circuit for transmitting signals from the outside of electronic equipment from an input terminal section to a processing circuit section. The signal transmission circuit 3 comprises a transistor 4, a first resistor 5 and a second resistor 6 as constitution parts which relate to signal transmission, other than the capacitor 1 and the printed wiring board 2. The printed wiring board 2 includes input wiring 7 connected to the input terminal section, output wiring 8 connected to the processing circuit section, and an insulation board 9 on which the input wiring 7 and the output wiring 8 are formed.
A base terminal of the transistor 4 is connected to the input wiring 7. An emitter terminal of the transistor 4 is grounded. A collector terminal of the transistor 4 is connected to the output wiring 8. The first resistor 5 is connected in parallel between the input wiring 7 and the emitter terminal of the transistor 4. One terminal of the capacitor 1 is connected to the input wiring 7. The other terminal of the capacitor 1 is grounded. To the collector terminal of the transistor 4, a predetermined reference voltage is supplied via the second resistor 6.
At the time of designing the signal transmission circuit 3 of the related art shown in FIG. 12, a constitution that relates to signal transmission of the signal transmission circuit 3 is determined firstly, and the noise component eliminating capacitor 1 is added after that. Therefore, usually, the transistor 4, the first resistor 5 and the second resistor 6 are mounted on a surface of the printed wiring board 2 on which parts are mounted, and the noise component eliminating capacitor 1 is mounted on a backside surface of the printed wiring board which is soldered. In this case, the input wiring 7 has a constitution that conductive patterns 11, 12 disposed to both the faces of the board 9, respectively, and are electrically connected by a via hole 13.
Since the via hole 13 has a larger resistance value than the conductive patterns, in a case where the via hole 13 exists on a first transmission path 16 from a connection point 14 with the input terminal section to a connection point 15 with the capacitor 1 in the input wiring 7, a resistance component of the first transmission path 16 acts as a resistor interposed between the input terminal section and the capacitor 1. Consequently, there is a possibility that the capacitor 1 does not function effectively for eliminating noise components.
Further, in the signal transmission circuit 3 of the related art, a second transmission path 18 from a connection point 15 with the capacitor 1 and a connection point 17 with the base terminal of the transistor 4 in the input wiring 7 is usually formed only by one via hole and two conductive patterns. In this case, a resistance component of the second transmission path 18 is lower than a resistance component of a resistor used for eliminating noise components. Thus, in the signal transmission circuit 3 of the related art, a constitution of using the resistance component of the second transmission path 18 for eliminating noise components in signals is not considered.